It is challenging to design a high-performance UHF RFID tag antenna on a liquid-filled bottle due to the high conductivity, high permittivity and the variety of liquids. A systematic way how to design a high-performance UHF RFID tag antenna on a liquid-filled bottle is presented. A simple design consisting of a folded dipole and a loop matching structure is first proposed for a tag antenna placed on a water bottle. A co-design approach is adopted to make the water and bottle surface become a part of the tag antenna to eliminate the significant influence of the environment. The measurement results at 915 MHz show that the reading range of the simple design can reach 8 m in the absence of the water bottle and reach 4.2 m when it is placed on a water bottle. The folded dipole on the water bottle is then optimized to improve the gain significantly, and a maximum gain of -5.54 dBi is achieved at 915 MHz. Furthermore, the impedance bandwidth for the tag antenna on the water bottle is greatly improved by increasing the number of loops in the impedance matching structure from one to three, and the -6 dB impedance bandwidth of over 100 MHz (simulation) or 84 MHz (experiment) is achieved. The reading range of 5.6 m is achieved when the tag antenna is placed on a water bottle at 915 MHz.
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